Ribavirin, a nucleoside analog, is one of the only broad-spectrum antiviral drugs available in the world, and it is a fundamental component of treatment for chronic Hepatitis C virus (HCV). Despite decades of clinical use, the exact mechanisms by which ribavirin inhibits HCV replication and causes its major dose limiting toxicity, anemia are unclear. Our lack of understanding of how ribavirin works and causes anemia represent fundamental gaps in our knowledge of the treatment of HCV. To optimize HCV cure rates and minimize anemia from ribavirin-based treatment, we need to understand this drug's pharmacology. In vitro, ribavirin depletes endogenous purines which likely explains its toxic and therapeutic effects, but this has not been investigated in humans. Through this application, Dr. Kiser will extend efforts on her K23 and explore the effects of ribavirin on endogenous purines. The specific aims for the study are to (1) compare the change in endogenous purine concentrations in the red blood and peripheral blood mononuclear cells of HCV-infected patients on ribavirin-based treatment across ITPA activity phenotype groups and (2) to determine associations between endogenous purine depletion and virologic response and anemia and the degree to which ITPA activity phenotype moderates these effects. To address these aims, we will measure endogenous purine concentrations before starting ribavirin-based HCV treatment and 4- and 12-weeks after initiating treatment and ribavirin triphosphate concentrations 4- and 12-weeks after initiating treatment in the peripheral blood mononuclear and red blood cells of 40 subjects in Dr. Kiser's K23 study and 170 subjects prospectively enrolled (and stratified by ITPA activity phenotype) through our Hepatology clinic. This study will increase our understanding of how ribavirin inhibits HCV replication and causes anemia and determine if there are differences in effect based on ITPA genetics. Additionally, while the LC/MS/MS assay developed through this grant will be immediately used to address the aims of this application, the technology can also be applied to investigations of the effects of HCV nucleoside polymerase inhibitors and nucleos(t)ide analogs used in the treatment of HIV, Hepatitis B, and cancer on endogenous nucleotide pools. PUBLIC HEALTH RELEVANCE: Millions of people around the world will develop hepatic complications from chronic HCV infection and our current therapies are inadequate because of their high incidence of adverse effects and suboptimal chances for viral cure. The work proposed in this application has high human health relevance since findings will shed light on the mechanism of action and toxicity for ribavirin. Ribavirin is an essential agent in the treatmen of HCV and many other life-threatening viral infections.